Due to variation in magnetic forces within rotating permanent magnet machines like generators or motors pulsations of torque occur. The torque pulsations occur at no-load and load situations, hereafter denoted cogging torque and ripple torque. These torque pulsations may lead to vibrations which can be harmful to the rotating machine and interconnected parts. Furthermore, the torque pulsations can create acoustic noise of low frequency which can disturb the environment. This problem can be even more pronounced in direct drive wind generators.
Reduction of torque can be achieved with different methods like optimizing the 2-dimensional shape of the magnet, shaping the stator/rotor tooth tips, displacing the magnets, conventional skew of the magnets or active damping via injection of current harmonics. Each method has its own drawbacks e.g. by magnet displacement, the magnet leakage flux is increased and thereby the output torque is reduced. Conventional magnet shaping in 2-dimension increases cost of manufacturing compared to conventional skew and needs accurate modelling tools like numerical FEM for optimizing the shape of the magnet.
Skew of the magnets is simple and inexpensive in general. However, conventional skew has several drawbacks like the distribution of magnet flux and thereby reduction of the magnet flux fundamental component and thereby reduction of the output average torque of the generator. Furthermore, for simplicity reasons of the magnet shape and fitting the magnet on a curved surface of the rotor, the skew is usually done in finite and few numbers of steps. As a result, more magnet flux leaks in the regions where two blocks are distant from one another, which is depicted by flux leakage vectors in FIG. 1. Consequently, more volume of magnets is required to provide the same average torque (i.e. magnets are not used efficiently).
Furthermore, since skewing in many steps may correlate with higher flux leakage, the skew is normally done in few numbers of steps and thereby the ripple or cogging torque minimization may not be fully effective for some designs.